I. Field of the Invention:
This invention relates to a thermal printing apparatus.
II. Background:
In conventional thermal printers, all heating elements of the printing head are supplied with power in blocks, each block being energized for a certain time. Each such element forms a "dot" which together create patterns perceived as characters. The capacity of the power supply is thus somewhat lowered. All the dot elements are divided into a plurality of blocks with, for instance, 256 elements per block. The capacity of the power supply is then calculated for the worst case condition (printing ratio=100%), and a certain number of blocks can be energized, based on the capacity of the supply. Despite the fact that the mean printing ratio is far lower than the actual one, power is supplied successively on a block-to-block basis because horizontal rule printing can be continuous. Thus, it is possible to increase printing speed somewhat, but this solution is inefficient.
In U.S. Pat. No. 4,447,819, the disclosure of which is expressly incorporated herein by reference, printing speed is increased by determining the number of blocks to be simultaneously energized according to the number of elements to be energized.
In addition, the printing paper used in these conventional fixed head type thermal printers has been limited in the prior art to predetermined widths. It would afford a considerable saving of paper if paper width could be selected according to the printing pattern. However, if such a selection of paper width were available, paper narrower than a particular printing pattern for use could be selected accidentally. In printing a pattern outside the paper width, the heat from the energized heat generating elements is not absorbed by the paper. Thus, the head can be damaged by the so-called "heating-an-empty-oven" phenomenon. Accordingly, such a conventional thermal printer has a disadvantage in that paper of the size fit for a particular printing pattern cannot be selected by the user.
In feeding printing paper in printers, constant speed feeding is normally used. However, it can be time inefficient to feed the paper at the same speed even when a blank portion passes the head in the paper feeding direction. As a result, attempts have been made to feed paper more quickly when a blank portion is detected is the paper feeding direction. If rolled paper is used as printing paper and thus quickly fed in this manner, the speed of rotation of the paper roll must vary as the paper feeding speed changes. If a large diameter paper roll is used, excessive tension may act on the paper because the speed of rotation of the paper roll depends on the paper feeding speed. This occurs because of the force of inertia of the paper roll. On the other hand, excessive paper will be let out when the paper feeding speed is reduced, thus possibly causing the paper to be caught between the paper feeding rollers and lodge therein. In order to eliminate such shortcomings, the paper roll shaft can be equipped with a driver for letting out the paper, and the driver should be so controlled as to follow changes in paper feeding speed. However, addition of such control tends to make printers expensive. Printers using a small diameter paper roll are far simpler in construction, as none of the aforementioned problems is posed. However, because a small diameter roll carries a small quantity of paper, it is necessary for the user to replace the paper roll frequently.
Thus, one object of the present invention is to increase mean printing speed by allowing the number of heating elements (the number of dots) to which power can be supplied simultaneously to be increased or decreased according to the printing ratio on each line. Another object of the present invention is to provide a fixed head type thermal printer wherein thermosensitive elements no longer in contact with the surface of paper as a result of paper width adjustment cannot be erroneously energized. Thus, the thermosensitive elements are prevented from being damaged by heat when narrow printing paper is used.
Another object of the present invention is to provide a relatively inexpensive printer mechanism for feeding printing paper quickly and smoothly at different feeding speeds, without causing excessive tension to be applied to the printing paper even if the paper roll diameter is large.